1. Field of the Invention
This invention generally relates to a surface acoustic wave (hereinafter simply referred to as SAW) filter having a SAW resonator, and more particularly, to a technique for protecting the SAW filter from electrostatic discharge (referred to as ESD).
2. Description of the Related Art
In recent years, wireless devices such as mobile telephones have rapidly been downsized, highly advanced, and highly improved in quality. A high-frequency circuit of the wireless device employs the filter having the SAW resonator. The filter structure includes a ladder-type filter and a multi-mode filter. The ladder-type filter is composed of multiple SAW resonators that are connected in a ladder structure. The multi-mode filter includes a double-mode SAW filter (referred to as DMS), for example. The SAW resonator includes a pair of comb-like electrodes that face each other on a piezoelectric substrate. This comb-like electrode is referred to as interdigital transducer (IDT). The comb-like electrode includes a bus bar and multiple electrode fingers that extend from the bus bar in the same direction. Gaps between the adjacent electrode fingers and those between edges of the electrode fingers and the bus bar that face the edges are extremely small. Therefore, in the case where static electricity is applied to an input terminal or an output terminal of the SAW filter, an electrostatic discharge occurs between the above-mentioned terminal and the ground. As a result, the SAW resonator, which is arranged between the terminal and the ground, is destroyed.
Japanese Patent Application Publication No. 6-224682 (hereinafter referred to as Document 1) discloses a structure for protecting the SAW resonator that determines the filter characteristics, by providing a sacrificial electrode for electrostatic breakdown. The sacrificial electrode for electrostatic breakdown is connected to the input terminal or the output terminal so as to be broken instead of the SAW resonator, in the case where the static electricity is applied.
However, there is the problem in that once the sacrificial electrode is broken, the SAW filter cannot be protected from the static electricity any longer. As a result, the reliability gets lower.